Spurious operation of the ECCS

In this case, cold (about 40°C) water enters the coolant circuit. Owing to this event, water subcooling at the fuel channel inlets will become larger, leading to an increase in the coolant density resulting from both a lower water temperature and a smaller void fraction in the fuel channels. 

Depending on the core conditions (power level), the reactivity feedback coefficient due to the coolant density may either be negative or slightly positive. Ingress of gas into the core is likely during injection by the hydroaccumulators. This also adds to the coolant density variations, which are bound to affect the reactivity. 

Considering the ECCS characteristics and operating procedures (including water delivery to one or both sides of the core), the initiating event should be analysed using a 3-D neutronics code coupled with multichannel thermohydraulic models. 

Primary attention should be paid in this analysis to the reactor power behaviour and to the performance of the CPS. 

These events include postulated breaks within the reactor coolant circuit ranging in size from small breaks up to guillotine breaks of about 800 mm in equivalent diameter, as well as failure of MSVs to close. 

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